When a single speed-governed engine drives a load, the governor sets the throttle at a setting which produces an engine torque output sufficient to maintain the speed commanded by the speed signal supplied to the governor. It does so by comparing the commanded speed and a detected engine speed and responding to any error by adjusting the throttle. The throttle adjustment changes the engine torque output and, therefore, corrects the speed to eliminate, except for "speed droop," the error.
When two or more engines drive a common load, as is often the case in marine propulsion systems, the virtually inevitable differences between the responses of the engine governors and other parts of the engines to speed signal and load changes result in different throttle settings and unequal division of the load among the engines. The engines all run at the same speed, that speed substantially (i.e., subject to "speed droop") matches the command speed, but each governor has no way of detecting whether the speed of the engine it controls is achieved due to the torque of that engine solely or some disproportionate share of the total torque of the multiple engines.
It is, of course, desirable that the load be shared equally by the engines, and engine load-sharing control systems are available. In multiple diesel engine marine propulsion systems, the load-sharing control systems generally used operate on the assumption, which is usually a reasonable one in practice, that if the fuel racks of the engines are set within about one percent of each other, the torque outputs of the engines are equal. Accordingly, the state-of-the-art control systems involve monitoring and comparing the fuel rack positions of the engines and altering the speed signals to the governors in accordance with any differences between the fuel rack settings.
The load-sharing control systems for multiple diesel engine marine propulsion are usually pneumatic. The fuel rack position of each engine is monitored by a fuel rack position detector (a mechanical-pneumatic transducer) which produces an output pressure proportionate to the fuel rack position. Each fuel rack signal is conducted through an orifice to a reservoir, the purpose of which is to produce a time delay between the time that a fuel rack change occurs and the time that the signal indicative of that change is processed in the control system. In a typical system, the reservoir is simply a long length, say about 30 feet, of small-diameter tubing. The fuel rack signals then enter a comparator which compares them and produces an error signal indicative of the difference between the fuel rack settings.
One of the engines is selected as a master or lead engine. The lead engine receives a speed signal which, in most systems, is not altered by the load-sharing control. The other engine or engines receive a modified speed signal produced by algebraically summing the fuel rack error signal and the master engine speed signal.
When the fuel racks of a master and a slave engine do not correspond, the modified speed signal received by the slave engine governor produces an adjustment of the slave engine fuel rack tending to make it coincide with the position of the master engine rack. Most systems have a very high gain--a small error produces a large change in the speed signal supplied to the slave engine governor. The high gain (i.e., small proportional band) tends to move the slave engine fuel rack past the desired position coincident to that of the lead engine fuel rack, and the slave engine fuel rack will "hunt." If a lower gain is applied in the system so that the slave engine fuel rack is brought more slowly toward the desired coincident position, the error between the fuel rack positions eventually disappears, but it takes a long time and the application once again of the unaltered main speed signal causes the governor of the slave engine to respond by moving the slave engine fuel rack away from the desired coincident position. Accordingly, the fuel rack moves back and forth in some band between the desired coincident position and the undesired non-coincident position, an oscillatory form of "hunting". The orifices and reservoirs reduce the frequency and magnitude of oscillation of the slave engine fuel rack but also reduce the responsiveness of the control system.
Present control systems, therefore, reduce the average discrepancy between the torque outputs of the master and slave engines but do not eliminate discrepancy because when the error tends to be eliminated, the characteristics of the governors/engines which led to the error in the first place produce a response which recreates the error. Under some conditions encountered in normal operation, it takes several minutes for the control system to reduce a relatively large separation between engine outputs due to the delay between the time when a fuel rack change occurs and the time of response of the control system. The orifices which produce the delay tend to become clogged with dirt, thus producing larger and longer separations between output torques as time passes from servicing of the system.